The present invention relates to the field of interferometry, in particular the construction of an electrostatically-controllable, tunable Fabry-Perot interferometer. This type of apparatus may, for example, be used as an interferometric filter.
Documents EP-668 490 and EP-693 683 describe electrostatically-controlled integrated Fabry-Perot interferometers.
This type of interferometer is also described in an article by M. Blomberg et al. entitled "Electrically Tunable Micromachined Fabry-Perot Interferometer in Gas Analysis" published in "Physica Scripta", vol. T69, pages 119-121, 1997 and also in an article by M. Viitasalo et al. entitled "Carbon Dioxide Sensor based on Micromachined Fabry-Perot Interferometer" published in SENSOR 97, Poster A7.36, pages 193-198.
These documents describe an optical resonant cavity machined in a silicon substrate. The cavity consists of a layer of air of thickness .lambda./2, where .lambda. is the working wavelength, machined between two parallel dielectric mirrors. One of the mirrors is movable while the other is fixed. Each mirror is fitted with control electrodes which may consist of deposits of highly-doped conducting silicon layers. Applying a potential difference between the control electrodes draws the movable mirror towards the fixed mirror, thereby altering the dimensions of the air gap and thus the resonance wavelength.
In the apparatus described in EP-668 490 the electrode of the movable mirror is annular and the perimeter of the mirror is machined to make it thinner, thereby rigidifying the center and improving the plane displacement of the useful optical zone in the center.
The apparatuses described in these documents use control electrodes fitted to the two parallel mirrors. Electrostatic command is achieved directly by applying a voltage across these electrodes. This technique requires electric contacts to be made with two conducting surfaces positioned facing one another, thereby increasing the complexity of the apparatus and the procedure required to construct it. In particular the contact with the upper mirror is difficult to achieve and requires local stacking of highly doped layers. Great care must be taken to insulate the various active components and avoid the possibility of short-circuits.